.flow-colorful {
    max-width: 600px;
    height: 150px;
    background: linear-gradient(to right, red, orange, yellow, green, cyan, blue, purple);
    animation: hue 6s linear infinite;
}
@keyframes hue {
    from {
        filter: hue-rotate(0deg);
    }
    to {
        filter: hue-rotate(360deg);
    }
}